Magnetic resonance (MR) imaging is a preferred form of medical imaging for several reasons, among them the high resolution of the resulting imagery (independent of wavelength), and its lack of side effects.
Over the past decade, considerable effort has been spent, and considerable progress has been made, in this field. However, existing MR techniques are still poorly suited for certain imaging tasks. One of these is in angiography of the extremities.
MR angiography (MRA)--the imaging of the circulatory system--is difficult in the extremities for a variety of reasons. One is that the subjects being imaged--veins and arteries--become progressively smaller at locations remote from the heart. Small features are difficult to discern in existing MR angiography techniques because signals corresponding thereto are masked in the final image by noise signals. Little can be done about noise per se; where improvement is possible is in increasing the desired signals so that noise's masking effect is reduced. The figure of merit by which this aspect of an MR imaging system's performance is measured is "signal to noise ratio," or SNR.
Another difficulty with any MR imaging of the extremities--especially the legs--is their size. To complete a MR examination of a patient's leg, the leg must be imaged at a variety of different locations, necessitating repeated repositioning of the patient. A typical examination requires three to six different patient positions. Such repositioning prolongs the examination, with attendant increase in cost and patient inconvenience.
The original technique for MR angiography of the extremities, and the technique still most popular for imaging of the thigh, is use of a whole-body receiving coil. Disadvantages of this method include the time spent repositioning the patient for each station, limited SNR, and limited spatial resolution.
Various attempts have been made to address the foregoing limitations of the body coil. None, however, has proved wholly satisfactory.
One attempt to address such shortcomings is shown in U.S. Pat. No. 5,548,218 to Lu. In this system, three identical loop coils are positioned around a patient--one around the pelvic region; one around the thighs, and one around the calves. These coils are driven in phase quadrature with associated planar coils, yielding some small improvement in SNR. However, the overall performance is disappointing.
Applicants believe the poor performance of Lu's system is due to its use of large, widely spaced coils to image from the pelvis to the feet. The weak signal emitted by precessing atoms--the phenomenon on which MR relies--diminishes with the cube of distance. So a change in distance from 2 cm. to 4 cm. (between the sensing coil and the precessing source) results in an eight-fold reduction in the signal strength received--nearly 10 dB. (This relationship does not hold for distances very near the sensing coil.) The large distances over which Lu's coils receive signals thus grossly impairs the SNR of the resulting imagery. Further, the large size of Lu's coils increases their noise pick-up, without a commensurate increase in their pick-up of desired MR signals (i.e. coil segments remote from a desired imaging site contribute little to the MR imagery of that site). Finally, the planar coils Lu employs to achieve quadrature operation suffer from the SNR drawbacks of all planar coils.
Another approach to MR imaging of the extremities is shown in U.S. Pat. No. 5,500,596 to Grist et al (also described in Alley et al, "Development of a Phased-Array Coil for the Lower Extremities," Magn. Reson. Med. 34:260-267 (1995)). Four coils are used in this arrangement: two horizontal and two vertical, forming an "I-beam" construction. The two horizontal coils span the length and width of both legs--one positioned above the legs and one positioned below. The vertical coils are both disposed between the patient's legs. This orthogonal coil arrangement was apparently adopted to reduce the effects of mutual inductance between the coils. In another embodiment, each horizontal coil is replaced with two coils, yielding a total of six coils.
Again, the Grist et al system suffers in SNR performance. Its use of four (or six) coils provides somewhat better performance than Lu. However, because the coils are planar, and because they span only three of the four sides of a patient's leg, substantial portions of the leg are still quite remote from any coil, impairing overall SNR. And, like Lu, each coil is large and spans a large area, again degrading SNR.
In "Magnetic Resonance Arteriography of the Pelvis and Lower Extremities," Magn. Reson. Quarterly, 9(3):152-187 (1993), Schiebler et al employed three overlapping coils on a cylindrical plastic form to image portions of a patient's leg. Adjacent coils were rotated 90 degrees from one another to minimize mutual coupling. Schiebler's arrangement, however, is unable to image an entire leg without repositioning because the coils span only 58 cm. Moreover, Schiebler's SNR suffers due to larger than necessary spacing between the coil and the leg.
In "Phased Array Coils for Upper Extremity MRA," Magn. Reson. Med. 33:224-229 (1995), Monroe et al compared three coil arrangements for MR angiography of the arm. One arrangement comprised three overlapping coils mounted along four inch PVC piping. A second comprised three overlapping coils mounted on acrylic plastic that was slightly curved by heating. The third comprised three overlapping coils mounted on a flat piece of acrylic. Monroe concluded that the latter arrangement was preferable. He criticized the first as not fitting all patients, and criticized the first two as providing only a 10-20% improvement in SNR over the third--an improvement he regarded as minimal.
What Monroe apparently compared, however, was the best SNR of each configuration. SNR, however, is a function of location. The best SNR is obtained at points relatively close to the receiving coil, i.e. close to the skin. At deeper locations, the SNR is necessarily worse. In many applications of MR imaging, applicants believe the important metric is not how good is the best part of the imagery, but how good is the worst.
Each of the foregoing approaches employs multiple coils. The use of multiple coils, more generally, is discussed in Roemer et al, "The NMR Phased Array," Magn. Reson. Med. 16:192-225 (1990) and Hayes, "Volume Imaging with MR Phased Arrays," Magn. Reson. Med. 18:309-319 (1991).
An important application of MR angiography of the extremities is in identification of distal run-off vessels in the leg and the foot. (See, e.g., Owen et al, "Magnetic Resonance Imaging of Angiographically Occult Runoff Vessels in Peripheral Arterial Occlusive Disease," N. Engl. J. Med. 326:1577-1581 (1992); Carpenter et al, "Magnetic Resonance Angiography of Peripheral Runoff Vessels," J. Vasc. Surg. 16(6):807-13 (1992); McCauley et al, "Peripheral Vascular Occlusive Disease: Accuracy and Reliability of Time-of-Flight MR Angiography," Radiology, 192:351-357 (1992); and Owen et al, "Symptomatic Peripheral Vascular Disease," Radiology, 187(3):627-35 (1993)). None of the earlier-described coil arrangements is satisfactory for this application. The techniques that have met with the most success have used a small transmit-receive linear extremity coil. Such a coil provides high SNR, but overlap of adjacent imaging segments is required because of signal loss at its boundaries. Depending on the patient's size and precision of coil positioning, three to five stations are required to image from the foot to the knee.
Accordingly, there remains a need for coil arrangements that allow faster MR angiography of the extremities, with better spatial coverage, and with improved SNR.
In accordance with a preferred embodiment of the present invention, the foregoing and other disadvantages of existing MR coils for the extremities are overcome. In the illustrated embodiment, an MR receive coil array includes six telescopically arranged coil units, spanning the length of the extremity from the inguinal ligament to the foot. The coil in each unit encircles the extremity, providing good SNR from all sides and for deep structures as well. The tapered shape of the array conforms generally to the patient anatomy, minimizing the sensing distances, further enhancing SNR. A low ratio between the volume imaged and the aggregate coil conductor length further contributes to high SNR. A multiplicity of tuning capacitors makes the array relatively insensitive to detuning by differently-sized patients.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.